Using a lithium battery on a Razer Orochi instead of two AAs

For those who don’t know the Razer Orochi [razerzone.com], here is the very short review:
The good:
– Very well built, good components
– Lots of buttons, cool features and configuration options
– USB / Bluetooth hybrid
– Very comfortable

The bad:
– Bluetooth power saving is too fast (after 20 or so seconds there is a momentary but noticeable lag to start moving again, didn’t notice this problem in microsoft bluetooth mice)
– Even with this agressive power saving the battery life is very bad (as in comically bad)

Regarding the first problem, the only way to fix this is probably changing the bluetooth module firmware (beyond my current capabilities) and after a while you kind of stop noticing it.

Regarding the battery life problem, like most people, I was using AA rechargeable batteries and the whole “take batteries out, recharge them, put them in again” is kind of inconvenient. The solution? A rechargeable lithium cell, of course!

How I did it:
First I bought a test 14500 cell on ebay, 2USD with shipping to Portugal included!
Here it is, next to a AA for size comparison:

Now the charge controller, I had a couple of AnalogicTech AAT3681 charging ICs so I made the eagle part library and etched a very simple PCB based on the datasheet:

Those 3 diodes are there as a very simple way to lower the voltage from 4.2V of the maximum Li cell to around 3.3V, closer to two new AAs (not the best way but very quick and cheap🙂 )

Now I just wired everything:
– The USB and battery ground are the same, this makes things easier as it will also be the charger ground.
– +5v from USB to charger input
– Charging current set to around 80mA (the charging IC is really small, I didn’t felt very confident with higher charging currents)
– The 3 diodes are in series between the Litium cell and the previous battery in
– The status pin of the charger is wired to a orange LED that I’ve placed near the mouse status LEDs

Closing time, battery in one side and controller board in the other:

And voilá! When I connect the mouse via USB, if the battery voltage is under the threshold, the charge starts: